Use Rayon to load datasets for indexing using multiple threads.

Created by: adamreichold

Let's do this after all as this reduces the time window in which the index does not consistently capture the datasets which might lead to broken links from the search results into the datasets.

Performance does improve measurably:

strategy	caches	wall time	CPU utilization
serial	cold	5min	20%
serial	hot	10s	250%
parallel	cold	1min	100%
parallel	hot	5s	350%

This is on a CODE-DE VM with 4 cores and 380k datasets. Caches were dropped via echo 3 > /proc/sys/vm/drop_caches.